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WEC Projects, one of the leading EPC contractors in the water sector in South Africa has had many years to perfect one of their products that has proven to add massive value to municipal and mining clients. The product is called the “Model R” packaged sewage treatment plant. Don’t be fooled by the words “packaged” – these systems are designed to treat relatively large capacities, from 350kl/day up to 3MLD (although WEC Projects is equally proficient in treating the smaller capacities too).

The Model R plant consists of 2 concentric round tanks with the inner tank being the clarifier and the outer tank the reactor. The outer tank is divided into different zones that allow for specific biological nutrient removal processes to occur, which allows the plant to treat effluent to the highest standards required in South Africa.

If you are a municipal client, the Model R plant may be just what you require to expand your existing bulk sewage treatment facilities capacity as is can be added as a separate module that either utilises some the existing infrastructure or is completely independent. For example, the existing inlet works and disinfection infrastructure can be used for effluent treated by an “add on” Model R plant. This helps keep your infrastructure upgrades cost effective.

The Model R packaged sewage treatment plant is also implemented as the main sewage treatment plant for small communities all over the African continent. It is a flexible system too, in that the plants’ tanks can be constructed from mild steel which is protected from corrosion utilising the latest cutting-edge epoxy coatings, or from concrete.

The basic process description for the Model R is as follows:

Raw sewage enters the plant through the head of works which consists of a screenings facility to remove non-organics and grit removal systems

Sewage flows into a buffer tank to absorb peak flows that may occur.

The activated sludge reactors are normally configured Modified Ludzack-Ettinger (MLE) process for de-nitrification, which is the preferred process to treat low alkalinity sewage.

The process can further be upgraded to a full nutrient removal capability by including an anaerobic section for phosphorus removal. The biological nutrient removal capability is based on the three stage Phoredox process configuration. The operation of the activated sludge reactor is based on a micro-organism population that is cultured in the system by natural selection. The micro-organism culture mainly consists of bacteria that utilise the organic matter in the sewage as substrate in the presence of dissolved oxygen. The organic matter is removed from the sewage, i.e. purified by this utilisation process. The utilised organic matter is transformed into bacterial mass which has a natural tendency to flocculate under the conditions maintained in the activated sludge system. The content of the activated sludge reactor is generally referred to as the mixed liquor. In a well-operated plant the mixed liquor consists of a clear liquor portion with a distinguishable, easily settle-able, biological floc. This floc can now be readily removed from the suspension by gravity settling. Separation is achieved in the clarifier from where settled biological floc is returned to the reactor.

As an improvement on the basic activated sludge process, the biological nutrient removal capability is achieved by adding an anaerobic compartment at the start of the process to stimulate enhanced biological phosphorous uptake. The anaerobic compartment is followed by an anoxic compartment for nitrogen removal by de-nitrification. The de-nitrification process reduces oxygen requirements through the utilisation of oxygen from the nitrate ion and reintroduces alkalinity to the treated effluent as an added bonus (making treated effluent less corrosive). The reactor contents need to be aerated in order to maintain acceptable dissolved oxygen levels to sustain the required biological processes. Dissolved oxygen is supplied by a system consisting of mechanical blowers (duty/standby), air distribution pipes and fine bubble membrane diffusers which are located at floor level in the reactor.

The solid/liquid separation step occurs in the clarifier. The clarifier is designed to allow settlement of the biological floc to a central hopper below, while the clarified supernatant liquor is decanted at the surface via a purpose designed effluent weir. The settled sludge is pumped back to the reactor section (also referred to as the sludge return or return activated sludge, or RAS), while the clarified effluent is routed to the subsequent disinfection process unit.

The treated effluent still carries pathogenic organisms that may vary from a virus, bacteria to any other type of pathogenic micro-organism. Final treatment, therefore, includes a disinfection step. Disinfection is achieved by dosing a disinfectant agent into the treated effluent stream at an appropriate rate.